Beacuase it's an older coin widely accepted by many people over the world. But it may be changes as people start using other coins.Like Monero on dark web. Where BTC volume reduce and Monero volume increased.
I don't think Bitcoin being the oldest cryptocurrency is the reason why its worth more than the other cryptocurrencies. The market price of Bitcoin is determined by people with huge number of bitcoins. They either increase the supply rate of bitcoin or decrease the supply rate, which affects the market worth of bitcoin.